Binder for producing articles from particulate materials

ABSTRACT

The disclosure relates to a binder, and the method of formulating the said binder, which is suitable for shaping parts from metallic and/or ceramic particles by injection molding. The binder comprises materials that are mainly thermoplastics, each of which having a percentage that is determined by its thermogravimetric analysis (TGA) profile and a weight loss versus highest binder removal rate from a green body by progressive heating. The removal of this binder from the green body is performed within a much shorter period of time than those published in the prior art. An example binder comprises 40-70% HDPE, 18-30% Paraffin wax, 10-25% microcrystalline wax and 2-5% stearic acid. Another binder comprises 35-65% PP, 23-35% paraffin wax, 10-25% microcrystaline wax and 2-5% stearic acid.

This is a divisional of application Ser. No.07/935,818 filed on Aug. 26,1992 now U.S. Pat. No. 5,332,543.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a binder used in shaping articles fromparticulate materials for manufacturing high precision, high density andhigh integrity engineering parts from particulate materials, such asmetal powders or particles, ceramic powders or particles or the like byinjection molding or similar processes, and the method of formulatingthe binder.

2. Art Background

Injection molding processes have increasingly been used to form articlesfrom particulate materials. This process enables the formation of smallpads with more complex geometry at even lower cost than conventionalforming processes.

Classically, particulate materials are mixed with a binder and theninjection molded into a desired configuration which is generally calleda green body. The binder contained in the green body is removed bythermal degradation prior to sintering. The body is sintered to obtainthe desired part with 90-96% density of the theoretical.

The binder plays a key role in the injection molding process. As acarrier, it facilitates flow and packing of the padiculate materialsinto the mold cavity and holds the particles in the shape of the moldafter the part is ejected. When the green body is successfully formed,the binder finishes its function as a carrier and needs to be removedfrom the .green body prior to sintering. The removal of the binder isperformed by means of thermal heating, or solvent washing, orcombinations thereof. This removal has to be fast, without creating anydefects in the green body configuration or leaving behind any residueswhich hinder densification of the body during sintering.

The selection of a binder is generally based on its flowcharacteristics. The slow binder removal process required in thisprocess remains the major drawback of the process despite the disclosureof various binders in the art. Much effort has been directed towardsreducing the debinding time through change of debinding techniques. Thisinvention permits a reduction in debinding time of thermal degradationthrough the formulation of binder systems which are not disclosed in theprior art.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of this invention to provide a novelbinder for powder injection molding that is superior to the binders ofthe prior art.

It is another object of the present invention to provide a method forbinder formulation which is solely based on thermogravimetric analysis(TGA) to determine the quantity of each binder constituent.

It is still another object of the present invention to provide animproved binder suitable for injection molding obtaining high packingand removal by thermal degradation such that it does not create anydefects in the molded article.

It is still a further object of the present invention to provide aprocess of binder removal by progressively heating the green body thatcontains the said binder such that it is faster than the prior art andit does not damage the part integrity.

These and other objects, as well as the scope, nature and utilization ofthe invention will be apparent from the following summary and detaileddescription of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of an ideal profile of the weight loss of the binderversus the degradation temperature. The degradation temperature beginsabove the processing temperature and is in the range of 200° to 500° C.

FIG. 2 is a graph of a debinding heating up profile for the trial binderformulation of the present invention.

FIG. 3 is a graph of a thermogravimetric analysis profile of the binderof Example 1.

FIG. 4 is a graph of a thermogravimetric analysis profile of the binderof Example 2.

DETAILED DESCRIPTION OF THE INVENTION

A binder generally contains multiple components that are usuallythermoplastics and some additives like stearic acid for wetting andlubrication. Binders formed from various combinations of these materialsor ingredients have been published. However, all the binders in theprior art suffer from a common problem, namely an excessively long timeis required for their removal. The inventors have found that a bindercan be so formed that it offers both better injectability at a higherthan normal powder loading and reduction in debinding time.

In accordance with one aspect, the present invention provides a methodof forming the binder and the said binder suitable for injectionmolding. In the other, the present invention provides a process ofbinder removal which involves thermal degradation of the said binder andachieves total removal in shorter time without creating any defects inthe molded article than the prior art.

In the prior art, a binder contains, generally, a wax and/or fatcomponent, which has a lower melting temperature and degrades at a lowertemperature than the second component, the thermoplastic, which beginsto degrade only after the first component has been fully removed fromthe green body. To this end, in the prior art the debinding process hasto be performed by providing for an extensively and excessively longholding period at the low temperature end, thus rendering anunnecessarily long debinding time.

The present binder comprises thermoplastics such as high density PE andPP, and waxes and acids. Differing from the binders published in theprior art, the present binder is so formed and the composition is sochosen that the size of the plateau on the degradation curve between thewaxes and polymers is minimized or eliminated. Thus, the elimination ofholding at the low temperature for a long time and the reduction oftotal debinding time are achieved. The present binder is formedfollowing the procedures below:

First of all, an ideal profile of the weight loss of the binder versusthe degradation temperature, as shown in FIG. 1, is designed for thebinder to be formed. The degradation temperature begins above theprocessing temperature and is in the range of 200° to 500° C.

Secondly, a series of thermogravimetric analysis tests are carried outon the selected, commercially available thermoplastics, waxes andsurfactant. In this case, high density polyethylene (PE), polypropylene(PP), ethylene vinyl acetate (EVA), polystyrene, Paraffin Wax,microcrystalline wax, PE wax and stearic acid are chosen.

Thirdly, the selection of binder components is performed according tothe thermogravimetric analysis curves and the degradation temperaturerange of the-chosen material, and an appropriate amount of the chosenmaterial is so determined that the thermogravimetric analysis curve fromthe formed binder resembles the ideal thermogravimetric analysis curve(FIG. 1) designed for the binder.

Finally, a debinding heating up profile is established for the trialbinder formulation on the basis of the thermogravimetric analysis ofeach individual component, which is shown in FIG. 2.

The inventors have found that the composition of a multiple-componentbinder can be formulated according to the above explained method. Theideal weight loss versus temperature curve (FIG. 1) is designed takinginto consideration the following: the vapor pressure generated by thatamount of binder at the specified temperature does not create anydefects such as blister or bloating in the green body. The lower limitof the temperature range may vary depending on the temperature forinjection molding. A general rule is that there should not be any orshould only be a minimum of controlled binder loss during injectionmolding. Otherwise, there will be problems in volume loading controland, therefore, a loss of control of part dimensions during sintering.Moreover, the binder loses its recycle ability so that heavy waste ofraw materials occurs. The upper limit of the temperature range isspecified based on the fact that at temperatures higher than this limitpolymer materials used for binder decomposed completely and that at evenhigher temperatures sintering starts.

With the desired profile shown in FIG. 1, formulation of a binder startswith the selection of candidate binder component materials. Generally,materials that meet binder requirements are waxes and thermoplastics. Inorder to formulate a binder that has a weight loss versus temperatureprofile similar to that in FIG. 1, available binder materials arestudied by thermogravimetric analyses (TGA). The resulting TGA profilesprovide a basis for the selection of the binder component and thecalculation of the composition of a binder formulation. The TGA profileof this trial binder may be different from that in FIG. 1 andoptimization of the formulation by changing the percentages of thebinder components is generally necessary. It may occur that the TGAprofile has a plateau as shown in FIG. 3. The existence of such aplateau is not desirable because it makes debinding time unnecessarilylong as nothing is evaporating during the heating at the temperaturerange of the plateau. To minimize or eliminate such a plateau, it isdesirable to add into the binder another component which degrades at thetemperature range of the plateau.

A binder that has a weight loss versus temperature profile in FIG. 1allows a fast heating profile of FIG. 2 to be used for binder removalfrom the green body containing the said binder. The initial heating upto the temperature 10° to 50° C. above the highest melting point of thebinder component is carried out at a rate of 300° to 500° C./h.Retention at this temperature for some time, typically 1 to 2 hours, isto relieve the residual stresses induced by injection molding, as wellas to initiate the creation of microchannels so that during thesubsequent heating the vapor generated by the binder can freely come outof the green body. This retention is required for the continued fastsubsequent heating up without causing any blistering, bloating, slumpingor distortion. The rate of weight loss at any specific temperature ismade to be high as possible without creating any internal defects in thepart. The profile is relatively flat initially since the microchannelsare very fine. As the temperature increases, the slope of the profile isgetting deeper because those microchannels are now enlarging. A muchhigher binder removal rate is achievable when some 60% of the binder hasbeen removed. The progressively accelerated heating profile shown inFIG. 2 enables the total binder removal to be performed within less than9 hours.

The present invention is described in greater detail with reference tothe following examples.

EXAMPLE 1

Polyethylene, paraffin wax, microcrystalline wax and stearic acid wereselected as candidate binder components. A binder was formulated thathad the composition as follows:

    ______________________________________                                        MATERIAL         W.T. %                                                       ______________________________________                                        HDPE             50                                                           PW2              20                                                           MC WAX           25                                                           STEARIC ACID      5                                                           ______________________________________                                    

The TGA profile of the binder is shown in FIG. 3.

A test piece part, which incorporates thick and thin sections, cores,cantilevered portion, etc., was injection molded using feedstock thatcontains the above formed binder and carbonyl iron powders in aproportion of 35:65 by volume. Binder removal from the green part wascompleted using the heating profile shown in FIG. 2. The total debindingtime was approximately 8 hours and the part retained the shape, free ofany defects.

EXAMPLE 2

Polypropylene, paraffin wax, microcrystalline wax and stearic acid wereselected as candidate binder components. A binder was formulated thathad a composition as follows:

    ______________________________________                                        MATERIAL         W.T. %                                                       ______________________________________                                        PP               45                                                           PW0.5            25                                                           MC WAX           25                                                           STEARIC ACID      5                                                           ______________________________________                                    

The TGA profile of the binder is shown in FIG. 4.

The same test piece as used in Example 1 was injection molded usingfeedstock that contains the binder and carbonyl iron powders in aproportion of 36:64 by volume. Binder removal from the green part wascompleted using the heating profile shown in FIG. 2. The total debindingtime was approximately 9 hours.

While the present invention has been described with a certain degree ofparticularity with reference to the embodiment and examples, manymodifications and variations will immediately become apparent in thelight of the above teachings. It is therefore the intention that theappended claims be interpreted as broadly as possible in view of theprior art to include all such variations and modifications.

What is claimed is:
 1. A binder used for shaping articles fromparticular materials comprising multi-component thermoplastic materials,each of which has an appropriate weight percentage so that the TGAprofile of the compound emulates the ideal TGA curve of FIG.
 1. 2. Thebinder materials in claim 1, wherein said multicomponent thermoplasticmaterials are selected from waxes, thermoplastic polymers and saidbinder material further comprise additives like stearic acid.
 3. Thebinder materials of claim 1 comprising 40-70% HDPE, 18-30% Paraffin wax,10-25% microcrystalline wax and 2-5% stearic acid.
 4. The bindermaterials of claim 1 comprising 35-65% PP, 23-35% paraffin wax, 10-25%microcrystalline wax and 2-5% stearic acid.
 5. The binder as set forthin claim 1, comprising 50% polyethylene, 20% PW2, 25% MC wax and 5%stearic acid.
 6. The binder of claim 1, comprising 45% PP, 25% PW0.5,25% MC wax and 5% stearic acid.